A map whose keys are derived from an other map. The keys are derived only when requested, which
make it possible to backup potentially large maps. Implementations need only to overrides baseToDerived(BK) and derivedToBase(K) methods. This set do not supports null key,
since null is used when no mapping from base to this exists. This
class is serializable if the underlying base set is serializable too.

This class is not thread-safe. Synchronizations (if wanted) are user's
reponsability.